Low Cost InGaAlN Based Lasers

ABSTRACT

A method and structure for producing lasers having good optical wavefront characteristics, such as are needed for optical storage includes providing a laser wherein an output beam emerging from the laser front facet is essentially unobstructed by the edges of the semiconductor chip in order to prevent detrimental beam distortions. The semiconductor laser structure is epitaxially grown on a substrate with at least a lower cladding layer, an active layer, an upper cladding layer, and a contact layer. Dry etching through a lithographically defined mask produces a laser mesa of length l c  and width b m . Another sequence of lithography and etching is used to form a ridge structure with width w on top of the mesa. The etching step also forming mirrors, or facets, on the ends of the laser waveguide structures. The length l s  and width b s  of the chip can be selected as convenient values equal to or longer than the waveguide length l c  and mesa width b m , respectively. The waveguide length and width are selected so that for a given defect density D, the yield Y D  is larger than 50%.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional under 35 U.S.C. 120 of copending U.S.application Ser. No. 12/171,286, filed Jul. 10, 2008, which is adivisional under 35 U.S.C. 120 of U.S. application Ser. No. 11/509,015,filed Aug. 24, 2006, now U.S. Pat. No. 7,408,183, which claims thebenefit under 35 U.S.C. 119(e) of U.S. Provisional Application No.60/710,882, filed Aug. 25, 2005, the disclosure of which is incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to semiconductor diode lasers and, morespecifically, to low-cost InGaAlN based lasers with etched facets.

BACKGROUND OF THE INVENTION

InGaAlN diode lasers are of interest as light sources for a number ofapplications; for example, in high-density optical storage, displays,printing and biomedicine. In many devices and systems associated withthese applications, laser sources are needed that are capable ofproviding an output beam with high wavefront quality. Furthermore, thewidespread use and commercial success of many of these systems anddevices depend on the ability to provide them at low cost. Consequently,high manufacturing yields and low cost are critical requirements for thelight sources needed to construct such systems and devices.

Semiconductor diode lasers based on epitaxially grown layers of at leastan n-type lower cladding layer, an undoped active layer with quantumwells and barriers, a p-type upper cladding layer, and a highly p-typedoped contact layer, have been fabricated from In_(x)Ga_(y)Al_(z)N,where 0<=x<=1, 0<=y<=1, 0<=z<=1, and x+y+z=1. These lasers are able toemit in a range of wavelengths that span at least from violet toblue-green wavelengths. Lasers of this type have been fabricated anddescribed in the prior art, see for example, S Nakamura, et al., “TheBlue Laser Diode: The Complete Story”, Springer-Verlag, 2000, but suchlasers face many challenges in meeting the requirements of highmanufacturing yield and low cost, high reliability, and high opticalquality of the output radiation.

The substrate materials that are currently available for the epitaxialgrowth of InGaAlN-based laser-active layers lead to unique problemswhich present substantial obstacles to achieving high manufacturingyield and low cost. For example, available substrates cause unusuallyhigh defect densities in the laser-active material layers and, inaddition, make it very challenging, if not impossible, to use mechanicalcleaving for the formation of laser mirrors due to the mechanicalproperties of the substrate material. Substrates made of SiC andSapphire have been used for the fabrication of InGaAlN lasers, but thesematerials do not permit lattice-matched growth of the InGaAlN layers,and result in very high defect densities, low manufacturing yield andreliability concerns. Recently, freestanding GaN substrates have becomeavailable for use in the fabrication of GaN lasers, as described inUnited States Patent Application Publication No. US 2003/0145783 A1 ofKensaku Motoki, et al, published Aug. 7, 2003. However, even when thehighest-quality GaN substrates are used, the laser active layers exhibita defect density of around 10⁵ cm⁻², which is several orders ofmagnitude higher than for typical commercial semiconductor lasers basedon other material systems. Furthermore, the size of these GaN substratesis currently limited to diameters of 2 inches, at most, and the cost isvery high. If a low cost is to be achieved, it is important to limit theimpact of the defect density on the laser fabrication yield so assignificantly to improve yield.

It is known that mirror facets can be formed on diode lasers by etchingtechniques, as described in U.S. Pat. No. 4,851,368, and in Behfar-Rad,et al, IEEE Journal of Quantum Electronics, volume 28, pages 1227-1231,1992, the disclosures of which are incorporated herein by reference.However, early work in etching GaN mirror facets did not result inhigh-quality facets. For example, etched surfaces that were desired tobe perpendicular to the substrate turned out at an angle from thevertical, as described in Adesida, et al, Applied Physics Letters,volume 65, pages 889-891, 1994, and the facets were too rough, resultingin poor reflectivity, as described in Stocker, et al, Applied PhysicsLetters, volume 73, pages 1925-1927, 1998.

Recently, a novel process that allows high quality mirror facets to beformed in a GaN material system has been described in U.S. applicationSer. No. 11/455,636, to Behfar et al, filed Jun. 20, 2006, and assignedto the assignee of the present application, the disclosure of which isincorporated herein by reference. As described in that application, itis very difficult to form multiple lasers of short cavity length on awafer through the use of conventional cleaving techniques because of themechanical handling that is involved in the cleaving operation. Inaddition, cleaving results in the simultaneous formation of mirrorfacets and the singulation of the wafer substrate into separate laserchips. Successful formation of cleaved facets is particularly difficultfor InGaAlN based lasers grown on GaN substrates, since the cleaving ofGaN crystals is more challenging than cleaving of the GaAs and InPsubstrates previously used for the mass-produced diode lasers utilizedfor CD, DVD and telecommunications.

On the other hand, use of the etching process described in applicationSer. No. 11/455,636 for the formation of laser facets permitsoptimization of the facet formation independently of the subsequentdevice singulation. In this process, lasers are fabricated on a wafer inmuch the same way that integrated circuit chips are fabricated onsilicon, so that the chips are formed in full-wafer form. The lasermirrors are etched on the wafer using etched facet technology (EFT), andthe electrical contacts are fabricated on the lasers. The lasers aretested on the wafer, and thereafter the wafer is singulated to separatethe lasers for packaging. Scanning Electron Microscope images of etchedAlGaInN-based facets show that a high degree of verticality andsmoothness can be achieved using the EFT process, which also allowslasers and integrated devices to be fabricated for a variety ofapplications having wavelength requirements accessible withAlGaInN-based materials.

The foregoing process for fabricating lasers can be summarized ascomprising the steps of lithographically defining a multiplicity ofwaveguide devices on a wafer having an AlGaInN-based structure andetching through the resulting mask to fabricate a multiplicity of laserwaveguide cavities on the wafer. Another lithographic step followed byetching is used to form laser facets, or mirrors, on the ends of thewaveguides while they are still on the wafer. Thereafter, electricalcontacts are formed on the laser cavities, the individual lasers aretested on the wafer, and the wafer is singulated to separate the lasersfor packaging. This method of etching the facets includes using a hightemperature stable mask on a p-doped cap layer of the AlGaInN-basedlaser waveguide structures on the wafer to define the locations of thefacets, with the mask maintaining the conductivity of the cap layer, andthen etching the facets in the laser structure through the mask using atemperature over 500° C. and an ion beam voltage in excess of 500V inCAIBE.

Selectivity between the etching of the semiconductor and the maskingmaterial is very important in obtaining straight surfaces for use inphotonics. High selectivity between the mask and the GaN based substrateis obtained by performing CAIBE at high temperatures. Large ion beamvoltages in CAIBE were also found to enhance the selectivity. The maskmaterials were chosen to withstand the high temperature etching, butalso to prevent damage to the p-contact of the GaN-based structure.

Particularly in the case of InGaAlN lasers, etching of the laser mirrorscan offer a number of important advantages for improving yield andreducing cost. For example:

-   -   (a) The laser cavity waveguide dimensions can be different from        the chip length dimension and can be optimized to maximize the        laser fabrication yield. By fabricating a waveguide of limited        length, the probability of a material defect occurring in the        laser active region is reduced and the fabrication yield is        increased.    -   (b) Redundant lasers can be fabricated on one semiconductor chip        to produce yield and reliability improvements.    -   (c) Surface emitting lasers with the laser cavity oriented        horizontally in the wafer plane can be fabricated by etching a        45° surface to direct the radiation upward out of the wafer        plane.    -   (d) Laser facet coatings for desirable reflectivity        modifications can be applied at the full wafer level prior to        device separation.    -   (e) Laser testing can be carried out economically at the        full-wafer level.    -   (f) Additional components such as photodiodes, lenses and        gratings can be monolithically integrated with the lasers.

The yield and cost of today's mass-produced diode lasers based on GaAsand InP substrates are not impacted by the substrate quality and cost.Substrates for these laser devices typically have defect densities ofabout 10² cm⁻² and are available in wafers of larger sizes of up to 6inches in diameter at a cost that is several orders of magnitude lowerthan that of GaN substrates. Both GaAs and InP have a zinc blend crystalstructure that facilitates the use of cleaving for both the formation ofthe laser end mirrors and the chip singulation, and cleaving is theprimary method used in volume production of these semiconductor lasers.In addition, in diode laser applications in areas such astelecommunications optical imaging is not a primary concern and therequirements on optical beam quality are more relaxed.

What is needed in order to produce InGaAlN lasers with high yield, lowcost, high reliability and good wavefront quality is a device designthat minimizes the occurrence of substrate-induced defects in and nearthe laser-active region, and provides an undistorted optical beam and amethod for fabricating such a laser device.

SUMMARY OF THE INVENTION

According to the present invention, an InGaAlN semiconductor diode laseris provided with laser mirrors formed by etching. The laser incorporatesspecial design features that decrease undesirable yield loss caused bysubstrate defects, provide lower device cost, improve reliability, andprovide an output beam with high optical wavefront quality. It has beenshown in the prior art that mirror or facet etching permits thefabrication of waveguide lengths as short as 3 micron. Additionaladvantages offered by laser facet etching, such as full-wafer testingand device integration have also been described in the prior art.However, the need for special choices of laser waveguide length andwidth to minimize yield reductions caused by material defects has notbeen recognized in the art, nor has the need for a specific geometry forproviding laser light of high optical wavefront quality been recognized.

Briefly, and in accordance with the present invention, a method andstructure for producing lasers having good optical wavefrontcharacteristics, such as are needed for optical storage, are provided.For these purposes, the geometry at and near the etched front facet ofthe laser is designed in such a way that an output beam emerging fromthe laser front facet is essentially unobstructed by the edges of thesemiconductor chip in order to prevent detrimental beam distortions.This requirement is in contrast to diode laser applications in otherareas, such as telecommunications, where optical imaging is not aprimary concern and the requirements on optical beam quality are morerelaxed.

In accordance with one aspect of the invention, a semiconductor laserstructure is epitaxially grown on a substrate with at least a lowercladding layer, an active layer, an upper cladding layer, and a contactlayer. Dry etching through a lithographically defined mask produces alaser mesa of length l_(c) and width b_(m). Another sequence oflithography and etching is used to form a ridge structure with width won top of the mesa in a preferred form of the invention, although it isunderstood that this invention is not limited to ridge laser structures,the etching step also forming mirrors, or facets, on the ends of thelaser waveguide structures.

The wafer is separated into single device chips using an appropriatesingulation process, such as sawing, cleaving after scribing, orlaser-based dicing. The length l_(s) and width b_(s) of the chip can beselected as convenient values equal to or longer than the waveguidelength l_(c) and mesa width b_(m), respectively.

Because etching is used for the formation of the laser mirrors, thelaser waveguide can be designed such that it is shorter than the lengthof the device chip and, specifically, a design can be chosen thatreduces the yield loss caused by the unusually high defect densitynormally encountered in the active layers of InGaAlN lasers. Theprobability of a material defect being located within the laser-activewaveguide region is related to its length l_(c) and effective intensityprofile width w_(l), and the defect density D. The yield Y_(D) forfabricating lasers without such a defect is inversely proportional tothis probability and can be expressed using Poisson statistics. Inaccordance with the invention, the waveguide length and width areselected so that for a given defect density D, the yield Y_(D) is largerthan 50%.

Particularly for applications requiring laser light with high opticalwavefront quality, it is important to design the geometry for the lasermirror, mesa and device chip in such a way that the laser output canemerge and propagate without significant obstruction by the chip edge.Therefore, it is desirable to create the front face of the device chipat a very small distance a from the front of the laser mesa from whichthe laser beam emerges. At the same time, singulation must be carriedout not too close to the high-quality mirror surface in order to avoiddamage. Since the dimensional accuracy of singulation is typically muchreduced compared to the lithographically controlled etching processes,it is desirable to choose a length l_(s) of the device chip that islonger than the length l_(c) of the laser mesa and to position thewaveguide such that a is small but not zero.

According to the present invention, the distance a between the edge ofthe chip and the bottom of the laser mesa and the height h between thecenter of the laser output beam and the top of the laser mesa must bechosen with particular attention to the beam divergence of the laserradiation emitted from the front laser mirror. In the directionperpendicular to the substrate, the radiation emerging from the frontlaser mirror has high intensity in the beam center and falls off over anextended distance away from the beam center. The vertical divergence ofthe emitted laser beam is typically characterized by itsfull-width-half-maximum farfield angle

marking the angular spread between the rays where the intensity isreduced to 50% of its center value. The edge of the device chip cancause a partial obstruction of the lower part of the propagating laserbeam. If this occurs, some of the light impinging onto the top surfacewill be reflected and interfere with the upper part of the laser beamand, together with the diffraction caused by the chip edge, can resultin undesirable distortions of the radiation profile and spatiallyvarying modulation of the laser intensity. For applications requiringhigh optical wavefront quality, it is important to minimize theobstruction of the laser light by the chip edge formed through thesingulation process.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing and additional objects, features and advantages of theinvention will become apparent to those of skill in the art from thefollowing detailed description of the invention taken with theaccompanying drawings, which are briefly described as follows.

FIG. 1A illustrates a singulated ridge-type semiconductor laser withetched facets perpendicular to the plane of the substrate.

FIG. 1B illustrates a cross section of a singulated ridge-type laser inthe direction along the laser waveguide.

FIG. 1C illustrates a cross section of a singulated ridge-type laser ina direction perpendicular to the laser waveguide direction.

FIG. 1D illustrates a singulated ridge-type semiconductor laser with anetched facet at 45° to the plane of the substrate.

FIG. 2 illustrates a top view of a section of a laser wafer prior toseparation into individual devices.

FIG. 3 shows the laser yield as a function of the laser waveguide lengthl_(c), wherein an effective width w_(l) of 4 microns was used for thecalculation and yield curves for two different defect densities of D=10⁵cm⁻², the lowest defect density reported for InGaAlN diode lasers, andD=10² cm⁻², a typical value for mass produced AlGaAs lasers, are shown.

FIG. 4 illustrates the emitter end of a singulated ridge-type laser as across section in the direction along the laser waveguide.

FIG. 5 illustrates a Gaussian intensity distribution of a laser beam inthe direction perpendicular to the propagation direction of the beam.

FIG. 6 a illustrates the emitter end of a ridge-type laser in thedirection with an auxiliary mesa as a cross section along the laserwaveguide.

FIG. 6 b illustrates a three-dimensional view of a device with a lasermesa of length l_(c), an auxiliary mesa of length l_(m) and a chip oflength l_(s) produced by singulation.

FIG. 7 illustrates four edge-emitting lasers fabricated on one chip.

FIG. 8 illustrates an optical pickup system comprising an InGaAlN laser.

FIG. 9 illustrates an optical storage system comprising an opticalpickup system and an InGaAlN laser.

DESCRIPTION OF PREFERRED EMBODIMENTS

Turning now to a detailed description of the invention, FIGS. 1A-1Cillustrate a diode laser chip 10, including a substrate 12 supporting anepitaxially grown and etched ridge laser 14 having end facets 16 and 18.The ridge laser is formed by etching of the laser end facets, as isknown in the art, with the present laser having a special designgeometry. In accordance with the invention, the semiconductor laserstructure 14 is epitaxially grown on substrate 12 with at least a lowercladding layer 20, an active layer 22, an upper cladding layer 24, and acontact layer 26. Dry etching through a lithographically defined maskproduces a laser mesa 30 of length l_(c) and width b_(m). Anothersequence of lithography and etching is used to form the ridge structure32 with width w on top of the mesa. While the laser schematically shownin FIGS. 1A-C and described above is a ridge-type design, it isunderstood that this invention is not limited to ridge laser structuresand also applies to other semiconductor laser designs.

As illustrated in FIG. 2, the substrate 12 may be in the form of aconventional wafer 40 on which is fabricated, by known masking andetching steps, multiple lasers 14, 14(a), 14(b), 14(c) . . . 14(n).After formation and testing of these lasers, the wafer is separated intomultiple single device chips 12, 12(a), 12(b), 12(c) . . . 12(n), usingan appropriate singulation process, such as sawing, cleaving afterscribing, or laser-based dicing along horizontal and vertical cleavinglines 42 and 44. The length l_(s) and width b_(s) of each chip can beselected to have convenient values equal to or longer than the waveguidelength l_(c) and mesa width b_(m), respectively, of each correspondinglaser 14.

By using an etching process for the formation of the laser and mirrorsor facets, such as facets 16 and 18, each laser waveguide can bedesigned to be shorter than the length of the corresponding chipsdevice. Specifically, a design can be chosen that reduces the yield losscaused by the usually high defect density found in the active layers ofInGaAlN lasers. The probability of a material defect being locatedwithin a laser-active waveguide region 46, such as that illustrated inFIGS. 1A and 1C, is related to the length l_(c) and effective intensityprofile width w_(l), of that region, and the defect density D. The yieldY_(D) for fabricating lasers without such a defect is inverselyproportional to this probability and can be expressed using Poissonstatistics:

Y _(D)=exp(−D*w _(l) *l _(c))  (1)

Although the yield is discussed herein below in terms of fabricatingInGaAlN lasers without any defects, it will be understood that it may bepossible to have a InGaAlN laser with a defect or defects in the cavitythat may function adequately for a particular application. However,reducing the defects in an InGaAlN laser in or near the active region 46has positive implications for laser yield and reliability.

The laser intensity profile in a direction perpendicular to thelongitudinal axis, or direction, of a laser waveguide 14, and in theplane of the substrate 12, is high in the center and falls off to thesides. Typically, the laser intensity profile can adequately bedescribed by a Gaussian distribution. For practical purposes w_(l) isdefined here as the width between the points where the intensity isreduced to 1/e³ of its center value.

It is noted that other effects will impact the total manufacturing yieldY while Y_(D) describes only the yield impact caused by material defectsof density D. FIG. 3 shows Y_(D) as a function of l_(c) for D=10⁵ cm⁻²and a width of w_(l)=4 micron. Yield values for laser lengths up to 1000micron, as typically used for semiconductor diode lasers, are shown.FIG. 3 clearly illustrates that an InGaAlN diode laser with a length of100 micron can be fabricated with a yield Y_(D) that is more than 5times higher than that for InGaAlN lasers with the typically used lengthof 500 micron.

FIG. 3 also illustrates typical values of Y_(D) for mass-produced 780-nmlasers made of essentially Al_(1-x)Ga_(x)As material layers, whichexhibit defect densities less than 10² cm⁻². In the case of InGaAlNlasers, substrate defects cause substantial yield degradation withincreasing laser length l_(c), whereas for 780-nm lasers they do not.Therefore, the design constraints implied by Eq. (1) do not have to beconsidered in the case of AlGaAs lasers, whereas they lead tosubstantial yield improvements for InGaAlN lasers.

According to the present invention, it is desirable to select thewaveguide length and width such that for a given defect density D, Y_(D)as determined by Eq. (1) is larger than 50%. The choice of a limitedwaveguide length has the additional benefit of reducing the totalinternal laser losses.

Particularly for applications requiring laser light with high opticalwavefront quality, it is important to design the geometry for the lasermirror 16, the mesa 30 and device chip 10 in such a way that the laseroutput light beam from facet 16 can emerge and propagate outwardlywithout significant obstruction by the edge 50 of the chip 10.Therefore, it is desirable to create the front face 52 of the devicechip at a very small distance a from the front facet 16 of the lasermesa 30 from which the laser beam 56 emerges (see FIGS. 1A and 1B). Atthe same time, the singulation of the chips must be carried out so thatthe cleaving lines 42 (FIG. 2) are not too close to the high-qualitymirror surface 16 in order to avoid damage. Since the dimensionalaccuracy of singulation is typically much reduced compared to thelithographically controlled etching processes it is desirable to choosea length l_(s) of the device chip 10 that is longer than the lengthl_(c) of the laser mesa 30 and consequently of the laser waveguide 32,as illustrated in FIGS. 1A and 1B, and to position the waveguide 32 sothat a is small but not zero.

According to the present invention, the distance a between the edge 50of the chip 10 and the bottom of the laser mesa 30 and the height hbetween the center line 54 of the laser output beam 56 and the top 58 ofthe laser mesa 30 must be chosen with particular attention to the beamdivergence 8 of the laser radiation 56 emitted from the front lasermirror 16 (FIGS. 1B and 4). In the direction perpendicular to the topsurface 60 of substrate 52, the radiation 56 emerging from the frontlaser mirror 16 has high intensity in the beam center 54 and falls offas a function of the distance away from the beam center. FIG. 5illustrates a Gaussian intensity distribution curve 70, whichillustrates and approximates the vertical radiation profile emitted bythe laser. The vertical divergence θ of an emitted laser beam 56 istypically characterized by its full-width-half-maximum (FWHM) farfieldangle

which marks the angular spread between rays having an intensity that isreduced to 50% of the center value of the beam 56. In FIG. 4, the beamboundaries 72 and 74 mark where the intensity is reduced to 50% of themaximum intensity in the center region 54, but a considerable portion ofthe total laser output propagates outside these boundaries. As can beunderstood from FIGS. 4 and 5, the edge 50 of the device chip 10partially obstructs the lower part of the propagating laser beam 56.When this occurs, some of the light impinging onto the top surface 58will be reflected and will interfere with the upper part of the laserbeam 56 and when this effect is combined with the diffraction alsocaused by the chip edge, the result will be undesirable distortions ofthe radiation profile and spatially varying modulation of the laserintensity.

For applications requiring a high optical wavefront quality, it isimportant to minimize the obstruction of the laser light 56 by the chipedge 50 that is formed through the singulation process. With

denoting the full-width-half-maximum (FWHM) far-field angle of theemitted laser radiation, a significant reduction of the beam obstructioncan be obtained by configuring the geometry of the chip 10 in accordancewith Equation (2):

$\begin{matrix}{\frac{h}{a} \geq {\tan( {\sqrt{\frac{2}{\ln \; 2}} \times \frac{\theta}{2}} )}} & (2)\end{matrix}$

Assuming a true Gaussian intensity profile at the chip edge, thegeometry design described by Equation (2) will reduce the laserintensity obstructed by the device chip to less than around 5%. Thisdesign can be implemented by forming the laser facets through asufficiently deep etch and by carefully controlling the singulationprocess that determines the distance a.

In an embodiment of the present invention, the accuracy required fordimension a can be relaxed by producing a relatively tall auxiliary mesaof length l_(m) as illustrated in FIGS. 6 a and 6 b. These figuresillustrate a chip device 80 fabricated on a substrate 12, in the mannerdescribed above with respect to FIGS. 1A-1C, and similar components areidentified by the same numerals. The chip device 80 differs from that ofFIGS. 1A to 1C in the provision of an auxiliary mesa 82 below, andsupporting, mesa 30 of the prior device. A number of techniques known tothose skilled in the art are available for the fabrication of such atall mesa. These processes do not need to produce the flat and smoothhigh-quality surface for mesa 82 that is required for the reflectivemirrors 16 and 18 at the ends of the laser waveguide. One processsuitable for producing the desired structure includes photolithographyto mark the position, length, and width of the auxiliary mesa 82 on topof the laser active layers 22, which are grown epitaxially on thesubstrate 52, as previously described. The material around the mesaboundaries is then removed to a depth of b. The laser mirrors 16 and 18are then formed by the process referred to above, this process alsodefining the length l_(c) and end positions of the laser waveguidephotographically to permit precise positioning of the laser front mirror16 relative to a forward edge 84 of the auxiliary mesa. If the distancea in FIG. 6 a is chosen to be relatively small, then a relativelyshallow etch depth is sufficient to assure that the ratio h/a meets therequirement of Eq. (2), thereby preventing any objectionable obstructionof the laser beam by the edge of the auxiliary mesa.

After completing the lithography and etching steps required to producefully functional lasers on a wafer, the wafer is separated intoindividual devices by positioning the singulation line 42 a suitabledistance a_(s) away from the edge of the auxiliary mesa, as illustratedin FIGS. 6 a and 6 b. In order to avoid undesirable beam obstructions,the distance a_(s) needs to be chosen so that:

$\begin{matrix}{\frac{h + h_{s}}{a + a_{s}} \geq {\tan( {\sqrt{\frac{2}{\ln \; 2}} \times \frac{\theta}{2}} )}} & (3)\end{matrix}$

It is important to understand that the fabrication of the tall auxiliarymesa with height b readily leads to h+h_(s) being large, whichfacilitates achieving a high ratio (h+h_(s))/(a+a_(s)) even withrelatively large values for a_(s). This relaxes the degree of accuracyrequired for positioning the singulation line at the front of the laser.Since common singulation methods are not photolithographicallycontrolled the described fabrication of an auxiliary mesa can improvethe manufacturing yield and reduce device cost.

Although the singulation surface has be shown to be flat in FIGS. 1A-Cand FIGS. 6 a-b, if a singulation surface is not used for providingoptical feedback to the laser cavity, it may have roughness and bejagged without detrimental effect on laser threshold or efficiency.

The description of the laser structure has so far focused on the frontof the laser device, which provides a laser beam to a system or devicemaking use of the output laser light from facet 16. As illustrated inFIG. 1B, the distance p between the rear laser mirror 18 and the rearchip edge 88 can be made large since there are usually no stringentrequirements for the optical beam quality any laser radiation emergingfrom the back mirror, or else the device is designed so that nosignificant radiation emerges from the back end.

In the embodiments of the invention described above, the laser frontmirror 16 is formed by etching the facet surface in a direction that isessentially perpendicular to the lasing direction, with the lasingdirection being essentially parallel to the plane of the substratewafer. However, as illustrated in FIG. 1D, surface emitting lasers canbe formed by providing an angled etched front facet 16′, which directsthe emitted light 56′ from the laser in a direction perpendicular to theplane of the substrate. Such laser structures are referred to ashorizontal-cavity-surface-emitting-lasers or HCSELs. In the case of theHCSEL, the beam shape of the emerging radiation 56′ is no longerimpacted by singulation and the distance a is not as critical as thatdescribed above. If desired, a lens 89 may be provided above the topsurface of the laser, as described in copending U.S. application Ser.No. 10/963,739, filed Oct. 14, 2004, and application Ser. No.11/037,334, filed Jan. 19, 2005, both of which are assigned to theassignee of the present application.

As discussed above and illustrated in FIGS. 1A and B, the etching of thelaser mirrors permits fabrication of short waveguide lengths l_(c), andthe chip length l_(s) can be selected independently of l_(c). In orderto reduce cost, it is advantageous to reduce the chip length l_(s) andwidth b_(s) as much as possible while choosing dimensions large enoughto keep handling difficulties during singulation, bonding and packagingat tolerable levels.

In another embodiment of this invention, the waveguide length l_(c) andlaser mesa width b_(m) can be chosen so that several lasers can beplaced on one chip of dimensions l_(s) and b_(s). When etching is usedfor formation of the laser mirrors, the functionality andcharacteristics of each laser can be tested prior to chip separation,wire bonding and packaging. This allows determination and selection ofthe laser with the most favorable properties for a specific targetedapplication. The fabrication of redundant lasers on each designateddevice chip permits beneficial yield improvements and performanceoptimization. Lasers can be edge emitters with perpendicularly etchedfacets or can emit vertically using a HCSEL structure. While it isunderstood that several redundant lasers can be placed on a chip in anumber of different geometrical arrangements and emission directions,FIG. 7 illustrates an example of a chip 90 having four redundant edgeemitting devices 92, 94, 96 and 98. As illustrated in FIG. 7, thewaveguide cavities 102, 104, 106 and 108, respectively, of eachindividual laser device is less than half of the chip length or width.In order to enhance yield, these several lasers are placed on a singlechip 90, with the four lasers depicted in FIG. 7 emitting light indifferent directions. Such a chip is packaged into, for example, aTO-type can and one of the lasers can be selectively wirebonded insidethe package to provide electrical current to the laser.

As is known, some freestanding GaN substrates have bands of low defectdensity GaN adjacent to bands of high defect density material. Lasercavities fabricated through cleaving are placed parallel to and withinthe low-defect-density bands. Short-cavity etched facet lasers can befabricated at any arbitrary angle to the low defect density bands andcan have their entire active region contained within the low defectdensity region.

An advantage of etched facet lasers is that coatings can be applied tothe front and back laser mirrors to modify their reflectivity.

One application of the low-cost InGaAlN lasers of the present inventionis in optical pickup devices used for optical data storage systems.While it is understood that such optical pickup devices and data storagesystems can vary in the details of their design, FIG. 8 illustrates thetypical components and functions of a pickup device generally indicatedat 120. In this device, an optical beam shaping system 122 collects andshapes the radiation emitted by laser 124. Shaping components andfunctions can include a lens for collimation, a prism forcircularization of the elliptical laser beam shape and a grating forproducing two sidebeams adjacent left and right to the main beam. Thelaser light then passes through a beam splitter 126 which may be apolarizing beam splitter followed by a quarter waveplate. A mirror 128directs the light to an objective lens 130, which focuses the light ontoa storage medium 132.

The light reflected from the medium returns through lens 130, isreflected off mirror 128 and is diverted by beamsplitter 126 to adetection system 134. The detection system 134 comprises a photodetector136 which is divided into multiple light-sensitive elements of ageometrical size and arrangement so that it produces electrical signalsindicative of the data encoded on the storage medium and servo errorsignals indicative of the lateral data tracking and vertical focusposition of the focused laser spot relative to the data. An opticalprocessing system 138 may be used to optically manipulate the light sothat the data and servo error signals are generated by photodetector136.

An actuator system 140, typically of electromechanical design, is usedto control the vertical and lateral position of the focused laser spoton the storage medium in response to the tracking and focus servo errorsignals.

FIG. 9 illustrates an optical storage system using an optical pickupdevice 150 with laser 124 and an optical disk medium 152, which isrotated by a motor 154. Data is encoded as an optically detectablematerial modification of the storage medium and arranged oncircumferentially oriented tracks around the disk medium. Each datatrack can be accessed with the optical pickup 150 by using anelectromechanical actuator system 156 to move and control the lateralposition of the focused laser spot. The optical data storage system canbe a read-only system that reads back data patterns that have beenpre-recorded on optical disks with a separate system. The optical datastorage system may also include the ability of writing data onto a diskmedium using the laser 124 and an electrical controller for modulatingthe laser intensity.

Although the present invention has been illustrated in terms ofpreferred embodiments, it will be understood that variations andmodifications may be made without departing from the scope thereof asset out in the following claims.

1. A chip, comprising: a substrate; a semiconductor of length l_(s) andwidth b_(s) on said substrate; at least one laser of cavity length l_(c)formed in said semiconductor; said laser having at least one etchedfacet; and wherein l_(c)<l_(s)/2.
 2. The chip of claim 1, whereinl_(c)≦b_(s)/2.
 3. The chip of claim 1, further including a second laserfacet formed by etching a surface essentially perpendicular to plane ofsaid substrate.
 4. The chip of claim 1, wherein said semiconductorcomprises GaN.
 5. The chip of claim 4, wherein said semiconductorfurther comprises of epitaxially grown InGaAlN layers.
 6. The chip ofclaim 1, wherein said semiconductor comprises InP.
 7. The chip of claim1, wherein said semiconductor comprises GaAs.
 8. The chip of claim 1,further including a second laser cavity.
 9. The chip of claim 8, furtherincluding a third laser cavity.
 10. The chip of claim 9, furtherincluding a fourth laser cavity.
 11. The chip of claim 2, furtherincluding a second, third, and a fourth laser cavity.
 12. The chip ofclaim 1, wherein said chip is incorporated in a package and wherein saidlaser receives electrical current though a wirebond.
 13. The chip ofclaim 8, wherein said chip is incorporated in a package and wherein atleast one of said first and second lasers receives electrical currentthough a wirebond.
 14. The chip of claim 1, wherein said at least oneetched facet is etched at or near a 45° angle to said substrate.
 15. Thechip of claim 14, including a lens formed above said 45° facet.
 16. Achip, comprising: a semiconductor of length l_(s) and width b_(s); atleast a first and second laser of cavities formed in said semiconductor;said first and second lasers each having at least one etched facet; andsaid first and second laser cavities having lengths shorter than l_(s);wherein said chip is incorporated in a package and wherein at least oneof said first and second lasers receives electrical current though awirebond.
 17. The chip of claim 16, wherein said first and second lasercavities further having lengths shorter than b_(s).
 18. The chip ofclaim 16, wherein said semiconductor comprises GaN.
 19. The chip ofclaim 16, wherein said semiconductor comprises InP.
 20. The chip ofclaim 16, wherein said semiconductor comprises GaAs.